tfq.differentiators.CentralDifference

Differentiates a circuit using Central Differencing.

Inherits From: LinearCombination, Differentiator

tfq.differentiators.CentralDifference(
    error_order=2, grid_spacing=0.001
)

Central differencing computes a derivative at point x using an equal number of points before and after x. A closed form for the coefficients of this derivative for an arbitrary positive error order is used here, which is described in the following article: https://www.sciencedirect.com/science/article/pii/S0377042799000886

my_op = tfq.get_expectation_op()
linear_differentiator = tfq.differentiators.CentralDifference(2, 0.01)
# Get an expectation op, with this differentiator attached.
op = linear_differentiator.generate_differentiable_op(
    analytic_op=my_op
)
qubit = cirq.GridQubit(0, 0)
circuit = tfq.convert_to_tensor([
    cirq.Circuit(cirq.X(qubit) ** sympy.Symbol('alpha'))
])
psums = tfq.convert_to_tensor([[cirq.Z(qubit)]])
symbol_values_array = np.array([[0.123]], dtype=np.float32)
# Calculate tfq gradient.
symbol_values_tensor = tf.convert_to_tensor(symbol_values_array)
with tf.GradientTape() as g:
    g.watch(symbol_values_tensor)
    expectations = op(circuit, ['alpha'], symbol_values_tensor, psums)
# Gradient would be: -50 * f(x + 0.02) +  200 * f(x + 0.01) - 150 * f(x)
grads = g.gradient(expectations, symbol_values_tensor)
grads
tf.Tensor([[-1.1837807]], shape=(1, 1), dtype=float32)

error_order A positive, even int specifying the error order of this differentiator. This corresponds to the smallest power of grid_spacing remaining in the series that was truncated to generate this finite differencing expression.
grid_spacing A positive float specifying how large of a grid to use in calculating this finite difference.

Methods

differentiate_analytic

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@tf.function
differentiate_analytic(
    programs, symbol_names, symbol_values, pauli_sums, forward_pass_vals, grad
)

differentiate_sampled

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@tf.function
differentiate_sampled(
    programs, symbol_names, symbol_values, pauli_sums, num_samples,
    forward_pass_vals, grad
)

generate_differentiable_op

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generate_differentiable_op(
    *, sampled_op=None, analytic_op=None
)

Generate a differentiable op by attaching self to an op.

This function returns a tf.function that passes values through to forward_op during the forward pass and this differentiator (self) to backpropagate through the op during the backward pass. If sampled_op is provided the differentiators differentiate_sampled method will be invoked (which requires sampled_op to be a sample based expectation op with num_samples input tensor). If analytic_op is provided the differentiators differentiate_analytic method will be invoked (which requires analytic_op to be an analytic based expectation op that does NOT have num_samples as an input). If both sampled_op and analytic_op are provided an exception will be raised.

This generate_differentiable_op() can be called only ONCE because of the one differentiator per op policy. You need to call refresh() to reuse this differentiator with another op.

Args
sampled_op A callable op that you want to make differentiable using this differentiator's differentiate_sampled method.
analytic_op A callable op that you want to make differentiable using this differentiators differentiate_analytic method.

Returns
A callable op that who's gradients are now registered to be a call to this differentiators differentiate_* function.

refresh

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refresh()

Refresh this differentiator in order to use it with other ops.